What are seismic waves, and what do they reveal about the Earth's structure?
P-waves and S-waves, how they travel through the Earth, the shadow zones, and the evidence they give for the layered structure and liquid outer core.
A focused answer to WJEC GCSE Physics topic 1.7 on seismic waves, covering longitudinal P-waves and transverse S-waves, how they travel and refract through the Earth, the shadow zones, and the evidence they give for the layered structure and a liquid outer core.
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What this topic is asking
WJEC wants you to describe P-waves and S-waves, explain how they travel through the Earth, and use the shadow zones as evidence for the Earth's layered structure. This is topic 1.7 Seismic waves in Unit 1 of WJEC GCSE Physics (3420).
P-waves and S-waves
How seismic waves travel through the Earth
The shadow zones
Evidence for the Earth's structure
The pattern of arrivals shows the Earth has a layered structure: a thin solid crust, a thick mantle, a liquid outer core and a solid inner core. The S-wave shadow zone shows the outer core is liquid; the way P-waves refract and the size of the shadow zones let scientists estimate the size of the core. Seismic waves are recorded by a seismograph (seismometer), which traces the ground movement against time.
Seismic waves are a good example of how scientists use indirect evidence to study something they cannot reach. No one can drill to the core, so its properties are inferred entirely from the way waves arrive at the surface. When detectors all around the world record the same earthquake, the missing arrivals map out the shadow zones, and the boundaries where waves suddenly speed up or change direction reveal the depths of the crust, mantle and core. This is why the model of the Earth's interior is so well established despite the core lying thousands of kilometres below the surface. The same idea, reading a structure from how waves pass through it, is used in medical ultrasound scans and in oil and gas exploration, where reflected waves reveal the layers of rock beneath the ground.
Try this
Q1. State which type of seismic wave is longitudinal. [1 mark]
- Cue. The P-wave (primary wave).
Q2. State what the S-wave shadow zone tells us about the Earth's core. [1 mark]
- Cue. The outer core is liquid, because it blocks S-waves.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC 20183 marksState the differences between P-waves and S-waves, and which can travel through liquids.Show worked answer →
A topic 1.7 question. P-waves are longitudinal and travel through both solids and liquids (1 mark). S-waves are transverse and travel only through solids, not liquids (1 mark). P-waves are also faster than S-waves, so they arrive at a seismograph first (1 mark). Markers reward the longitudinal-versus-transverse point, that only P-waves pass through liquids, and that P-waves are faster. A common error is to say S-waves travel through liquids.
WJEC 20214 marksExplain how the S-wave shadow zone provides evidence that the Earth has a liquid outer core.Show worked answer →
A topic 1.7 Explain question. S-waves are transverse and cannot travel through a liquid (1 mark). After an earthquake, seismographs detect S-waves over part of the Earth but there is a large region on the far side, the S-wave shadow zone, where no S-waves arrive (1 mark). The simplest explanation is that the S-waves have been blocked by a liquid layer, the liquid outer core, which absorbs them (1 mark). The size of the shadow zone also lets scientists estimate the size of the core (1 mark). Markers reward S-waves not passing through liquid, the shadow zone of missing S-waves, and the conclusion of a liquid core.
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Sources & how we know this
- WJEC GCSE Physics specification (3420) from 2016 — WJEC (2016)